Amplification system



Sept. 19, 1944.

J. P. FAY 2,358,630

AMPLIFICATION SYSTEM Filed April 21, 1943 --CABINET Inventor" James P. Fay,

His Attorny Patented Sept. 19, 1944 UNITED STATES PATENT OFFICE AMPLIFICATION SYSTEM James-P. Fay, Norwalk, Conn, assignor to General Electric Company, a corporation of New York Application April 21, 1943,.Serial No. 483,886

5.0laims. (01. 179-1) ing equipment, it may be desired to mount the My invention relates to amplification systems and particularly to such systems used in connection with sound reproducers and recorders, such as the loudspeakers of radio receivers and recording heads.

One of the cbjectsof my invention is to provide such an amplification system capable of maintainin constant sound output from the associated loudspeaker notwithstanding variations in impedance of the loudspeaker due to acoustic effects.

Another object of my invention is to provide such a. system in which the sound wave output is maintained independent of any resonance ef-- scription taken in connection with theaccompanying drawing in which the single figure represents an embodiment of my invention.

Referring to the drawing I have shown therein a two-stage audio frequency amplifier employing electron discharge devices I, 2' and 3, which is utilized to supply audio frequency voltages representing voice or music waves to a loudspeaker 4, or other device, such as a recordloudspeaker [into a number of different cabinets. Resonance eifects may occur in these cabinets which affect the impedance of the voice coil 8 of the loudspeaker and thereby affect the amount of energy supplied to the voice coil by 'the amplifier thereby tending to affect the volume of sound produced by the loudspeaker at the particular frequency at which the resonance effect occurs. Thus the output of the loudspeaker may vary in the diiferent cabinets.

- Such effects may also result from resonance efiects in the room in which the receiver is located, or in the mechanical structure of the speaker itself. These resonance effects may 4 either increase or decrease the impedance of the loudspeaker 8. Mechanical or acoustic resonance produces an, apparent increase in impedanceof the voice coil and an increase in sound output. Mechanical or acoustic anti-resonance produces an apparent lowering of the voice coil impedance and reduces the sound output. These efiects are particularly noticeable where the electron discharge devices employed-in the output stage of the receiver are of the high-impedance, constant-current type involving electron discharg devices of the pentode type. They are less noticeable where'these discharge devices are triodes'such as those indicated at 2 and 3 on thean objectionable de-' drawing but still exist to gree.

In accordance with my invention these effects are substantially eliminated by supplying a voltage from the output of the amplifiers 2 and 3 ing head. Audio frequency voltages suchas I those reproducedin the ordinary radio receiver may be supplied to the input of electron discharge device I through an audio frequency transformer 5. The output circuit from this amplifier is supplied through transformer 6 to the input circuits of the discharge devices 2 and 3, which may be connected inpush pull relation. The output from these discharge devices is supplied through a transformer 1' across the voice coil 8 of the loudspeaker 4.

The loudspeaker 4 may, of course, be mounted in the usual cabinet of the radio receiver which cabinet may, of course, house the remaining receiving equipment which may include the am plifier described. This cabinet is represented by the line III on the drawing bearing the legend "Cabinet." Y

Of course, in the manufacture of radio receivto the input of amplifier I which varies both in magnitude and phasein accordance with variations in the impedance of the voice 'coil 8. To efifect this control the voice coil 8 is connected as one arm of a Wheatstone bridge comprising'the arms 8, l3, l4 and IS. The arm I3 is an inductance equal to the normal inductance of the voice coil 8 of the loudspeaker. The arms l4 and I5 are suitable resistances of small value. The point between the arms l3 and i4 is grounded as indicated at IS. The point between the arms 8 and li is connected through conductor I! to a point between two cathode resistors l8 and I9, which are connected in series between the cathode of the discharge device I and ground.

Normally when the loudspeaker 4 is unaffected by any resonance effect which exist in the cabinet It or in the room in which the loudspeaker is located, the inductance of the coil 8 is vequal'to that of the coil is and the bridge is balanced.

Thus no voltage exists between ground and conductor ii. If a resonance effect occurs which increases the impedance of the coil 8, then the voltage at the frequency at which the resonance occurs appears on conductor l1 and is supplied across resistor I8 and hence between the cathode and control electrode of device i in series with the signal which is applied to the input of this is produced of opposite phase on the conductor l1 tending to render the discharge device regenerative at the particularly frequency of the antiresonance. This increases the amplification of the amplifier and hence the voltage supplied to the voice coil 8 thereby increasing the volume of sound produced by the loudspeaker to the value which would be produced in the absence of the anti-resonant condition. In this way the feedback of the amplifier I is varied in response to impedance of the voice coil and through a range loudspeaker independent of any resonant or anti-resonant mechanical or acoustic condition to which the speaker may be subjected. Thus the same amplifier system and speaker may be mounted in any one of a number of different cabinets without obpectionable change in the sound output due to resonance efiects in the cabinets.

While I have shown a particular embodiment of my invention, it will, of course, be understood that I do not wish to be limited thereto since different modifications may. be made, and I contemplate by the appended claims to cover any modifications which fall within the true spirit and scope of my invention.

, sufficient to maintain the sound output from the assaeso What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In combination, an amplifier, a sound reproducer connected to the output of said amplifier, and means responsive to variations in impedance of said sound reproducer to vary the gain of said amplifier to maintain constant output from said sound reproducer.

2. In combination, an amplifier, a sound reproducer connected to the output of said amplifier, and means responsive to variations in imduced by said loudspeaker, and means responsive to variations in impedance of said loudspeaker to control the voltage supplied by said amplifier to said loudspeaker'to maintain the sound output of said speaker independent of said resonance effects.

5. In combination, an amplifier having an output circuit and an input circuit, a Wheatstone bridge having two diagonals, said output circuit being connected across one of said diagonals and said bridge being normally balanced so'that substantially no voltage appears acros the other of said diagonals, a loudspeaker subject to impedance variations connected in an arm of said bridge whereby voltage appears across said other 40 diagonal having magnitude and phase varying in 

